Thanks! I've added that alternative now.
"They are drinking some water" was already accepted alongside "They are drinking water", but for the simple present, the alternative was missing.
I think both versions are acceptable -- some acts a bit like an indefinite plural article in English sometimes, in which case I wouldn't translate it in Greek.
Note the difference: λίγο is "a bit, a little" and is used with uncountable things, μερικοί/μερικές/μερικά is "a few, several" and is used with countable things. Λίγοι/λίγες/λίγα "few" is also possible.
So λίγο νερό could be used for "some water" if you want to specify that it's just a bit of water.
No. As a native speaker of English that is not right. Some is used as an indefinite article for uncountable or plural nouns. It is usually optional in English but compulsory in e.g. French (de l'eau). This should not be translated into Greek.
But some can also be used to mean 'a little bit of'. In this case you use λίγο or other variants mentioned by mizinamo. Of course if you do not know which is meant in English than you can choose the Greek translation.
It seems to me that many early learners are trying too hard to complicate these early lessons with extra stuff they think they know. But complexity should come naturally in later lessons. Sure the capitols make it easy, but we are getting familiar with simple words and structure. Sure there are other ways to answer with complexity, but these are first lessons. Just learn the lesson as given and move on. I'm happy to learn as I go.
For anyone looking for grammar tips, some units (especially the ones at the beginning of the course) have a Tips and notes section. If you're using the app you might not have access to them, but logging in to the website you'll see the link as you're clicking to start a lesson.